Like a physical address, Internet domain names are hierarchical (only a little more strict), so while your address might look like:
House name: 3 Willow Walk Town: Trumpton County: Trumptonshire Country: England
An Internet domain name looks like:
Host name: www Domain: gondolin Second level domain: org Top-level domain: uk
Which is represented as www.gondolin.org.uk.
As with physical addresses, the exact layout can vary. Sometimes there will be more parts to the address, so in the same way that houses can be divided into multiple flats or apartments, domains can be divided into multiple subdomains (there could be a domain name like "www.testing.gondolin.org.uk" for example). Sometimes there will be fewer parts, too - typically the larger the organisation, the shorter their domain name, ibm.com for example.
There used to be only a few top-level domains, but that changed in 2015, when a large list of new ones were added. The main (old, established) ones currently are:
Some of the more common new top-level domains are:
There are also internationalised domain names now, using non-latin characters (e.g. 谷歌 for China)
You may notice the word "historically" quite a lot, above. That's because as the Internet grew more popular, the original restrictions became harder to enforce (and possibly unnecessary). In the 1990s if you wanted a .net domain you had to prove you were a network provider, but now anyone can get one.
Useless trivia: The "uk" country code should be Ukraine and the United Kingdom should be using "gb" (Great Britain), but for historical reasons the United Kingdom uses "uk" and Ukraine ended up with "ua".
Second level domains aren't quite so restricted (you can have pretty much whatever you like under "com", "org" or "net"), although typically each country has its own version of "com", "org" and "net" and the actual domain lives under that (the UK has "co.uk" and "org.uk" for example)